1. Technical Field of the Invention
The present invention relates to an apparatus for controlling a Faraday rotation angle of a light beam passing through a garnet single crystal having a Faraday effect by applying external magnetic fields to a Faraday element composed of the garnet single crystal in two or more of directions and varying a synthetic magnetic field synthesized from the external magnetic fields. In particular, the present invention relates to a Faraday rotation angle varying apparatus capable of suppressing a variation in characteristics and also extending a variable range (typically, more than 90.degree.) of a Faraday rotation angle by specifying a displacement path of a synthetic vector of external magnetic fields with respect to a crystal orientation. The apparatus is useful for a polarization scrambler, an optical attenuator, and the like.
2. Related Art
An optical communication system or the like requires a polarization scrambler for varying a polarization direction of light continuously and periodically or an optical attenuator for controlling a quantity of transmitted light, in which a Faraday rotation angle varying apparatus is incorporated. The Faraday rotation angle varying apparatus is used to control a Faraday rotation angle of a light beam passing through a Faraday element by applying magnetic fields to the Faraday element in two or more of directions and varying a synthetic magnetic field synthesized from these magnetic fields. In general, a magnetic garnet single crystal having a Faraday effect is used as a Faraday element.
The basic configuration of a polarization scrambler is shown in FIG. 3. Light outgoing from an optical fiber 10 is collimated through a lens 12, to form parallel beams of light. The parallel rays of light pass through a Faraday element 13 and are converged at an incident end of an optical fiber 17 through a lens 15. The Faraday element 13 is applied with a magnetic field in the direction parallel to the optical axis by an electromagnet 19, and is further applied with a magnetic field in the direction perpendicular to the optical axis by a pair of permanent magnets 22a and 22b. The polarization direction of a light beam passing though the Faraday element 13 can be changed continuously and periodically by applying a fixed magnetic field larger than a saturation magnetic field of the Faraday element 13 to the Faraday element 13 from the permanent magnets 22a and 22b so as to turn the Faraday element 13 into a magnetic saturation state and then changing the polarity of a current supplied to a coil of the electromagnet 19 so as to change the polarity of a synthetic magnetic field synthesized from the applied magnetic fields.
The basic configuration of an optical attenuator is shown in FIG. 4, and one example of the structure thereof is shown in FIG. 5. Light outgoing from an optical fiber 10 is collimated through a lens 12, to form parallel beams of light. The parallel beams of light pass through a polarizer 14, a Faraday element 16, and an analyzer 18, and are converged at an incident end of an optical fiber 22. The Faraday element 16 is applied with a magnetic field in the direction perpendicular to the optical axis by an electromagnet 30 and is further applied with a magnetic field in the direction parallel to the optical axis by a pair of permanent magnets 26 and 28. Each of the electromagnet 30 and permanent magnets 26 and 28 has an aperture allowing light beams to pass therethrough. The quantity of transmitted beams of light can be controlled by turning the Faraday element 16 into a magnetic saturation state by the permanent magnets 26 and 28, and changing a current supplied to a coil of the electromagnet 30 so as to change a synthetic magnetic field. In the figure, a portion indicated by reference numeral 24 is a Faraday rotation angle varying apparatus.
An optical attenuator mainly includes a polarizer, a Faraday rotation angle varying apparatus, and an analyzer which are arranged in this order. The Faraday rotation angle varying apparatus includes a Faraday element which is generally added with an electromagnet for applying a magnetic field in the direction perpendicular to the optical axis to the Faraday element and permanent magnets for applying a magnetic field in the direction parallel to the optical axis to the Faraday element. The Faraday element is turned into a magnetic saturation state by the permanent magnets, and the direction of a synthetic magnetic field is changed by varying a current supplied to a coil of the electromagnet. The Faraday rotation angle is changed by such turning of the synthetic magnetic field, to change a quantity of the beams of light passing through the analyzer.
In each of the polarization scrambler and optical attenuator, the Faraday rotation angle is required to be changed depending on magnetic fields applied to a Faraday element (garnet single crystal) with a high repeatability. To meet such a requirement, as described above, the Faraday rotation angle is changed by applying a fixed magnetic field larger than a saturation magnetic field of the Faraday element to the Faraday element in the direction perpendicular or parallel to light beams so as to turn the Faraday element into a magnetic saturation state, and then applying, in such a state, a variable magnetic field to the Faraday element in the direction parallel or perpendicular to the light beams so as to change a synthetic magnetic field, to thereby change the direction of magnetization of the Faraday element. The reason why the Faraday element is turned into a magnetic saturation state is that if the Faraday element is in a magnetic unsaturation state, there occur deterioration of the extinction ratio and scattering of light by generation of magnetic domains, thereby degrading repeatability of the Faraday rotation angle depending on the applied magnetic fields.
The present inventors have prepared, on the basis of the above knowledge, polarization scramblers and optical attenuators in each of which a Faraday element composed of a garnet single crystal was set to be turned into a magnetic saturation state by a fixed magnetic field applied from permanent magnets. Each polarization scrambler thus prepared, however, presented a problem that a DOP (Degree of Polarization) is largely varied up to 20 to 30%. Each optical attenuator thus prepared also presented a problem that a magnetic field applied from an electromagnet, which is required for obtaining the maximum attenuation, is largely varied in a range of about 400 to 1000 Oe.
Further, there occurs the following problem. In the case where the direction of a synthetic magnetic field is changed from the direction parallel to the optical axis to the direction perpendicular to the optical axis so as to change the Faraday rotation angle from 90.degree. to 0.degree., the quantity of transmitted beams of light is changed from the maximum value to the minimum value when the angle formed between a polarizer and an analyzer becomes 90.degree.. However, since the fixed magnetic field is usually applied in the direction along the optical axis by the permanent magnets, a very large magnetic field must be applied in the vertical direction to change the direction of magnetization of the Faraday element into the vertical direction. In this case, whatever large magnetic field is applied in the vertical direction, it is theoretically impossible to change the synthetic vector of the magnetic fields to be directed 90.degree. (perpendicular) to the optical axis. Therefore, it has been regarded that the direction of magnetization of the Faraday element is not turned up to 90.degree., and consequently the Faraday rotation angle does not become 0.degree..